The present invention relates to internal drum recording apparatus. More particularly, the invention relates to apparatus for recording an image on a photosensitive recording medium, and to a method for carrying out the recording with high throughput.
Drum image-setters, generally speaking, comprise a drum on which a photosensitive recording medium is positioned, and beam generating means for generating a beam which is capable of recording an image on the medium. The recorded medium is then used in the printing process of the images previously recorded. In external drum image-setters the photosensitive medium is positioned in the outer surface of the drum. In contrast, in internal drum image-setters, with which the present invention is concerned, the medium is positioned on the internal surface of the dram.
FIG. 1 schematically illustrates a classical configuration of an internal drum recording apparatus. It includes mainly the drum 1 that supports the recording material 2, a guide beam 3 on which a carriage 4 is moving. The carriage 4 supports a deflecting rotating element 5, shown as a cube reflector in the figure, although other elements, such as a penta prism or a mirror, are also possible. It also supports a focusing lens 6. The light source assembly 7 is attached to the drum 1 or to guide beam 3, and emits a collimate beam 8 in the direction of the carriage 4.
It is well known that the direction of the collimated beam determines to what extent the imaged line will be a straight line.
When attempting to operate a multibeam internal drum scanner, two beams which are meant to record two lines, when in close proximity, originated by two non-parallel collimated beams will intersect if the non-parallelism is too high. This problem is schematically shown in FIG. 2, where the two beams from the source are indicated by 8 and 8xe2x80x2, and the resulting beams writing on the photosensitive material, as 9 and 9xe2x80x2, which image lines 100 and 100xe2x80x2. In any case, the slightest departure from parallelism in the collimated beams will result in non-parallel imaged lines.
Internal drum image-setters which work with a single imaging beam, tend to be limited in throughput because of mechanical limitations. As the available data rate increases with the increased performances of the screen processors, the only way to take advantage of this situation, when using a rotation speed of the rotating deflecting element that directs the light towards the light sensitive material. Increasing the rotation speed, however, is also limited by the technology available in the art.
Various schemes of internal drum image-setters, operating with more than one simultaneously recording beam, have been reported. Some are based on the use of a rotating element that is equivalent to mirror parallel to the rotation axis (U.S. Pat. No. 5,579,115). Others use a derotation element which spins in synchronization of the deflecting element, at half the speed; as described in U.S. Pat. No. 5,214,528. Others use two beams of different polarization (EP 483827). Another attempt to solve the problem is described in U.S. Pat. No. 5,764,381, in which the light sources inside the drum rotate. WO 97/42595 discloses a scheme for exposing light sensitive material in an image setter of an internal drum type means. U.S. Pat. No. 5,367,399 discloses still another scheme for a DRUM scanner.
All the attempts made in the prior art to solve this problem of internal drum recording apparatus have significant drawbacks. They generally involve complex solutions, and have limitations. For instance, providing a mirror parallel to the drum axis results in apparatus that is limited in scanning angle. Apparatus based on derotation requires extremely accurate and stable mechanical adjustments. Apparatus using double polarization is limited to two beams. Apparatus using rotating sources requires data transfer to the rotating sources, which is complicated and expensive.
Thus, the art has so far failed to provide a simple and efficient solution to the aforementioned problems.
It is therefore an object of the invention to provide apparatus which overcomes the aforementioned disadvantages of the prior art, and which permits to provide high throughputs of internal drum imaging apparatus.
Its is another object of the invention to provide apparatus based on the traditional internal drum scheme.
It is a further object of the invention to provide apparatus which, with minor modifications to an existing system, allows updating from single to multibeam operation.
It is yet another object of the invention to provide a method for operating a plurality of beams in an internal drum imaging apparatus.
Other objects and advantages of the invention will become apparent as the description proceeds.
The invention is directed to internal drum recording apparatus provided with a rotating deflecting element, comprising two or more beams of different wavelength, said two or more beams being simultaneously operable, and optical elements for bringing said two or more beams to a common optical path before they reach said rotating deflecting element.
According to a preferred embodiment of the invention the deflecting element has dispersing properties such that beams of different wavelengths will leave it at slightly different angles.
In a preferred embodiment of the invention the beams of different wavelengths are generated by laser diodes. Preferably but non limitativelyxe2x80x94the laser diodes are tunable laser diodes.
The apparatus of the invention comprises a beam combiner to combine the beams generated by the light sources. According to a preferred embodiment of the invention the beam combiner is a beam splitter. According to another preferred embodiment of the invention the beam combiner is a dichroic beam combiner. According to still another preferred embodiment of the invention the beam combiner comprises an optical fiber coupler.
In another aspect the invention is directed to a dispersing prism for use as a deflecting element in a multi-beam, multi-wavelength internal drum recording apparatus, comprising two coupled transparent optical elements between which a reflective surface is provided, said reflective surface being inclined at about 45 degrees with respect to the input beam, and wherein the input and/or output face(s) of the prism are tilted with respect to the optical axis.
According to a preferred embodiment of the invention there is provided a dispersing prism for use as a deflecting element in a multi-beam, multi-wavelength internal drum recording apparatus, comprising two coupled transparent optical elements between which a reflective surface is provided, said reflective surface being inclined at about 45 degrees with respect to the input beam, and wherein the input or output face of the prism has a cylindrical shape.
The invention also encompasses a prism assembly, comprising a prism the input face of which has a cylindrical shape, which is coupled to a lens having a refractive index different from that of the dispersing prism. Alternatively, the prism can be coupled to to a wedged prism. Illustrative and non-limitative examples of suitable lenses include cylindrical lens and wedge cylindrical lens.
The invention further provides a method for separating the imaged lines in a multi-beam, multi-wavelength internal drum recording apparatus, by tuning the source wavelength, said method comprising the steps of providing, for each desired resolution, a mask consisting of a reflective layer deposited on a transparent surface from which said reflective layer has been removed so as to form two parallel line segments positioned one after the other with an offset, the width of said lines being small compared to the optical spot size used, positioning behind the mask a light detector capable of detecting peak signals, and tuning independently at least one of the light sources so as to obtain the maximum peak signal. The invention further conveniently provides a method for operating with high throughput an internal drum recording apparatus provided with a rotating deflecting element, comprising providing two or more simultaneously operable beams of different wavelength, and bringing said two or more beams to a common optical path before they reach said rotating deflecting element.
According to a preferred embodiment of the invention the deflecting element has dispersing properties such that beams of different wavelengths will leave it a slightly different angles.